This application describes a research program to establish the mechanism and to characterize the protein components of the methotrexate transport system of L1210 mouse leukemia cell. Experiments are designed to investigate the possibility that methotrexate enters these cells via an anion exchange mechanism, to identify intracellular anions that participate in this process, and with the use of affinity-labeling agents to purify and characterize the membrane-associated binding protein which mediates methotrexate transport. The results will contribute to our understanding of how methotrexate and other antifolate drugs enter cells during chemotherapy, assist in the development of more effective drugs and regimens which use those agents, provide information on the nature of drug resistance that occurs via alterations in transport and how this type of resistance might be overcome, and to ascertain whether drugs directed against transport systems might provide an effective means for killing cells via folate starvation.